(a) Technical Field
The present invention relates to a microreactor, more particularly to temperature control inside the reactor and improvement of reaction rate and reforming performance thereby.
(b) Background Art
Hydrogen is widely used in petroleum and chemical industries mainly to produce ammonia through reaction with nitrogen. It is also used to obtain methyl alcohol through reaction with carbon monoxide and for cracking or desulfurization of heavy oil in petrochemistry. Recently, hydrogen is used as a reactant in the Fischer-Tropsch synthesis whereby a liquid fuel is synthesized from coal, natural gas or biomass. Also, hydrogen is an ideal fuel for a polymer electrolyte membrane fuel cell (PEMFC).
The main pathways for hydrogen production are thermal, electrochemical or biological. Among these methods, the thermal method, specially steam methane reforming, is the most economical, effective and widely used method. The steam methane reforming is accompanied by water-gas shift and reverse methanation reactions. Each reaction is as follows.Reaction Formula 1CH4+H2OCO+3H2,ΔH298=206 kJ/mol  1.CO+H2OCO2+H2,ΔH298=−41 kJ/mol  2.CH4+2H2OCO2+4H2,ΔH298=164 kJ/mol  3.
Since the steam methane reforming and reverse methanation reactions are endothermic reactions and the reaction rate of the water-gas shift reaction is relatively low, supply of heat from outside is required to continue the reactions. The reaction rate is also limited by the transfer of materials and heat inside the reactor. Accordingly, use of microchannels that have a large surface area per unit volume and thus exhibit superior transfer of materials and heat in a reactor is studied. An advantage of the microreactor is that it can be integrated with a heat exchanger. By doing so, heat can be supplied directly by inducing an exothermic reaction such as catalytic combustion of methane at the opposite side of a channel where the endothermic reaction occurs.
The reaction formula for the catalytic combustion of methane is as follows.Reaction Formula 2CH4+2O2→CO2+2H2O,ΔH298=−802 kJ/mol  1.
Control of temperature distribution inside the microreactor where the exothermic and endothermic reactions occur at the same time is of great importance. The reactor temperature is determined by the reaction rate of the endothermic or exothermic reaction and, conversely, the reaction of each reaction is greatly affected by the temperature. That is to say, the reaction rate and the temperature are closely related with each other.
The steam methane reforming and the catalytic combustion of methane exhibit different features in reaction rate. The reaction rate and temperature of the catalytic combustion of methane increase rapidly once the reaction is activated and the reaction is limited by mass transfer. Accordingly, the catalytic combustion of methane is already completed near the inlet of the microreactor. In contrast, the steam methane reforming proceeds relatively slowly and it is not completed with a conversion rate of 100% even at the outlet. Accordingly, if a catalyst is filled or coated throughout a combustion catalyst plate in the reactor as in the existing art, as shown in FIG. 1, the catalytic combustion of methane is terminated before the heat generated during the combustion is effectively transferred to the endothermic, reforming reaction and the heat remaining without being transferred to the reforming reaction rapidly increases the temperature in the reactor locally.
International Journal of Hydrogen Energy 37(2012) 13013 addressed the hot-spot problem of local temperature increase near the inlet of a microreactor where the steam methane reforming reaction for production of hydrogen and the catalytic combustion of methane for supply of heat occur adjacently owing to the difference in the heat of the endothermic and exothermic reactions based on computational analysis.
The high reactor temperature may result in deteriorated catalytic performance through thermal deactivation of the catalyst known as sintering and increases the possibility of detachment of the coated catalyst increases if the reactor is operated at high temperature. In addition, the high temperature and temperature gradient cause deterioration of reactor durability and increase deformation due to thermal stress.
Korean Patent Application No. 10-2006-0086062 discloses a device capable of reducing non-uniformity of temperature of a plurality of reactors while maintaining each reactor at a uniform temperature, wherein a long reaction flow channel is formed such that the temperature of the entire reaction flow channel is maintained constant and a heating part is provided between the plurality of reactors to uniformly heat them.
Korean Patent Application No. 10-1995-0006785 discloses a methanol reforming apparatus in which a reforming tube defined by coaxially arranged inner and outer cylinders filled with a reforming catalyst is provided to improve thermal efficiency by lowering operation temperature of the reforming apparatus as much as possible and to realize optimum combustion condition and temperature distribution, wherein a plurality of combustion catalyst layers of a honeycomb structure are installed inside the inner cylinder of the reforming tube, a fuel supply pipe is installed at the center of the combustion catalyst layer and a fuel vaporizing coil is installed inside the reforming tube to be connected with the reforming tube.
Korean Patent Application No. 10-2010-0042107 discloses a micro-macrochannel reactor to solve the problems of non-uniform distribution of reactants, pressure increase and deterioration of reaction activity caused by pressure change, which includes an upper end plate and a lower end plate engaged externally; a heat exchanging plate passing heat exchanging materials through a flow channel so as to perform heat exchange between the heat exchanging materials and reactants, products or a mixture that passing through a catalyst plate; the catalyst plate stacked with the heat exchanging plate and including a catalyst part accommodating a catalyst required for reaction such that the catalytic reaction occurs while the reactants pass through the catalyst part; and a supporting plate stacked with the catalyst plate and providing a flow channel so that the reactants can pass through the catalyst part of the catalyst plate, wherein the heat exchanging plate, the catalyst plate and the supporting plate are stacked between the upper end plate and the lower end plate.
Korean Patent Application No. 10-2004-0080918 discloses a system wherein hydrocarbons and air for combustion are mixed at thin plates so as to generate local heat for combustion and induce temperature increase such that combustion at a reactant inlet can be prevented and calories required for heating can be generated uniformly on the whole surface of the reactor.
In other words, the existing hydrocarbon reforming reactor for production of hydrogen has the hot-spot problem of local temperature increase caused by the difference in the heat of the endothermic, reforming reaction and the exothermic, catalytic combustion.
Throughout the specification, a number of publications and patent documents are referred to and cited. The disclosure of the cited publications and patent documents is incorporated herein by reference in its entirety to more clearly describe the state of the related art and the present invention.